New developments in fiber optic transmission systems indicate that frequency division multiplexing (FDM) is desirable to achieve high data rate communications. A limitation is placed on the technology by the fact that bandpass filters necessary to extract the optical signals from the carrier are only manufacturable using discrete fiber optics. This has inhibited the ability to achieve the goal of fully-integrated optics for frequency division multiplexing, in addition to making the components prohibitively expensive due to the required assembly of separately fabricated parts.
Optical RF bandpass filters can be created from optical waveguides by creating a resonant cavity formed of small lengths of optical waveguides, with length being one half the modulation wavelength, so that the RF signal will be extracted from the carrier. The RF bandpass filter is created out of the optical waveguide by deposition of mirrors at both ends. Thus far, only fiber optic technology has been used in the manufacture of RF bandpass filters relying on resonant cavities. Planar waveguides have not been used to form RF bandpass filters up to this point because absorption and scattering have been found to be too great to allow high "Q" resonators to be fabricated. The absorption and scattering found in the planar waveguides have been thought to be caused by surface imperfections at the walls of the waveguide. These factors lead to typical attenuation factors of about 10 dB/m for planar waveguides. The forced reliance on fiber optic-based bandpass filters has prevented the creation of monolithic arrays of optical filters.
It would be desirable to be able to fabricate optical RF bandpass filters using planar technology to allow the fabrication of an array of RF bandpass filters on a single substrate. The array of bandpass filters could be made with varying center frequencies to allow a single substrate to contain multiple bandpass filters. It is to this end that this invention is directed.